This invention relates to supported nanometer-sized catalytic crystal particle compositions of noble metals wherein the exposed faces of the nanometer-sized catalyst particles comprise predominantly crystal planes of the (111) type. The invention further relates to the method of preparing the nanometer-sized catalyst particles having predominantly (111) crystal planes as well as to the discovery of the particularly efficacious properties of these noble metal nanometer-sized particles in the catalysis of reactions such as hydrogenation, dehydrogenation and fuel cell applications
In commercial practice, a large number of chemical transformations are accomplished using heterogeneous catalysts where the active component of the catalyst is a metal, often deposited on a support material. An important class of catalysts are those which utilize noble metals including the platinum group of metals (platinum, palladium, rhodium, ruthenium, iridium, and osmium) as well as silver, gold and rhenium. In comparison to other catalytic materials, the noble metals often have especially high activity and selectivity and are, therefore, frequently preferred over other catalyst materials.
Because of their ability to facilitate the dissociative adsorption, surface reaction, and recombination/desorption of hydrogen, noble metals can catalyze a wide range of commercially important reactions, particularly the transfer, reaction or activation of hydrogen. These reactions and associated processes include various hydrogenations and related reactions such as methanation, carbonylation, hydroformylation, reductive alkylation, amination, hydrosilation, ammonia synthesis, oil or fat hardening and the like. Noble metal catalysis is also particularly useful in dehydrogenation reactions such as catalytic reforming, dehydrohalogenation reactions, steam reforming, partial oxidation and the production of gases including synthesis gas, hydrogen, carbon monooxide, carbon dioxide and the like. Noble metals are also used in important electrochemical processes such as fuel cell applications to carry out the dissociative adsorption of hydrogen and oxygen on the noble metal electrode, leading to the reaction of hydrogen with oxygen and the production of an electric current and by-product water.
Despite their wide-ranging effectiveness in catalytic chemical processes, noble metal catalysts have critical liabilities as the catalyst of choice for commercial chemical operations. Noble metals are extremely high cost and high catalyst attrition rates in a given chemical process can negate the choice of noble metals as a preferred catalyst for that process. Their high cost also requires that their superior catalytic activity be sufficiently high compared to other catalyst choices to justify their use. To ameliorate the cost disadvantage, those skilled in the art can employ noble metals catalyst as small particles whereby the overall efficiency of the catalyst is improved. This results in the greatest exposure of active noble metal surface areas for a given loading and offers the greatest noble metal utilization.
Nevertheless, even when methods are available to produce very small particles to improve efficiency and control catalyst costs, these particles are often unstable. The high surface energy of the small particles tends to cause migration and agglomeration of the metal particles such that a catalyst which initially comprises very small particles and high noble metal surface area can lose surface area. Improved methods are needed to prevent this migration and agglomeration of catalyst particles.
Another critical problem associated with noble metal catalysis is catalyst leaching by dissolution of noble metal into the surrounding liquid reaction medium. Because of the high cost of noble metals, leaching can be a very costly burden on overall process economics as it leads to a lose of catalyst efficiency and necessitates an enhanced catalyst recovery process Typical noble metal catalysts are known to have leaching rates of 5-10% per year or higher; that represents a substantial economic burden on any catalytic process. Accordingly, those skilled in the art have long sought improved methods to anchor noble metal particles on substrate surfaces in smaller particle sizes that will neither agglomerate nor experience prohibitive catalyst leaching rates.
Yet another problem with the current state-of-the-art for noble metal catalysts is the inability to properly control the active site distribution on the catalytic surface. For any particular catalyzed reaction, it is known that only certain types of surface active sites are, indeed, catalytically active and preferred for the selected catalyzed reaction. A key determining locus that defines the controlling characteristics of the surface active sites is the noble metal crystal face. Small metal crystals generally expose mainly low-order crystal faces, such as (100), (110), and (111). Normally, the most desired active sites for a given reaction are on only one of these faces so that a catalytically very active catalyst particle would be one that preferably contains a preponderance of that particular active crystal face. Unfortunately, prior art catalyst manufacturing methods produce catalysts containing crystal faces having a mixture of all of these preferred and non-preferred crystal faces. This reduces the efficiency of noble metal utilization and can also lead to non-selective reactions that are catalyzed by the non-preferred active sites on the crystal face. Therefore, the utility and efficiency of noble metal catalysts would be improved by introducing catalyst preparation methods that control the distribution of the crystal face exposition of noble metal particles to favor those configurations most effective in catalyzing the selected reaction.
Addressing the aforementioned problems to overcome them and provide an improved catalyst for selected types of reactions has led to the discovery that supported nanometer scale noble metal particles containing a preponderance of (111) type of crystal phase upon the crystal face of the catalyst particles are especially effective catalysts for reactions that fall within the general classes of hydrogenation or dehydrogenation reactions, including half-cell electrochemical reactions as carried out in hydrogen/oxygen fuel cells for the independent production of electricity. The support material for the noble metal catalyst is preferably a porous material such as porous alumina or carbon black.
The invention provides both an improved supported noble metal catalyst comprising nanometer scale particles applicable to selected reactions and a method for producing the improved catalyst. The particles of the invention preferably comprise noble metal particles of less than 5 nanometers, more preferably less than 2 nanometers. An especially important aspect of the invention is the discovery that the most effective crystal face of the noble metal particles useful in the selective reactions have a predominant exposure of the (111) type of crystal phase. Yet another important aspect of the invention is the finding that the nanometer scale noble metal particles are anchored to the surface of a supporting substrate in a way that prevents their subsequent migration and agglomeration. As a consequence of these properties, the catalyst of the invention is advantageously useful over the prior art for many important catalytic reactions. Depending on the application or reaction-type, these advantages include: (a) a higher activity derived from the increased noble metal surface areas of the extremely small crystallites containing the selective exposure of predominantly the (111) type of crystal phase upon the noble metal faces; (b) higher selectivity due to the selective exposure of the (111) type of crystal phase upon the noble metal crystal faces, (c) an improved catalytic stability and life due to the anchoring of the noble metal crystallites.
The method preparing the supported noble metal catalyst having nanometersized crystal particles with a preponderance of (111) type of crystal phase on the face of the noble metal particles includes the steps of:
preparing a solution of a noble metal salt and a metallo-organic sequestering agent;
treating the solution of sequestered noble metal with a reducing agent;
impregnating a catalyst support with the reduced noble metal solution;
drying the impregnated support; and
activating the catalyst by reducing the dried impregnated support to yield the nanometer-sized noble metal catalyst having a preponderance of 111 type of crystal phase on the face of the noble metal particles.
Depending on the metallo-organic sequestering agent employed to prepare the initial sequestered noble metal solution, treatment of the solution with a reducing agent in a second step may not be necessary and the noble metal sequestered solution may be used directly to impregnate the catalyst support prior to activation. Generally, before impregnating the catalyst support material, in the process of the invention it is not necessary to hydrogenate sequestered noble metal solutions prepared by reaction with small molecule metallo-organic sequestering agents.
As to the types of catalytic reactions to which the catalyst of the invention can be applied, hydrogenation is a preferred choice and includes the full scope of hydrogenation reactions as applied to acyclic and cyclic olefinic compounds, carbonyls, aromatic compounds, and oxides of nitrogen, sulfur, phosphorous, carbon and the like. Applicable catalytic dehydrogenation reactions especially include reforming of naphtha, cycliization of aliphatic compounds and alkane dehydrogenation to form alkenes. The catalyst is also useful in steam reforming of hydrocarbons, partial oxidation and the like.